THIN FILM ELECTRONICS WITH NOVEL MATERIALS: ZINC OXIDE AND 2D TRANSITION METAL DICHALCOGENIDES
Open Access
- Author:
- Gong, Yiyang
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 29, 2017
- Committee Members:
- Thomas Nelson Jackson, Dissertation Advisor/Co-Advisor
Thomas Nelson Jackson, Committee Chair/Co-Chair
Mauricio Terrones Maldonado, Committee Member
Ying Liu, Committee Member
Roman Engel-Herbert, Outside Member - Keywords:
- Thin Film Transistor; ZnO; Plasma Enhanced Atomic Layer Deposition; High Voltage Transistor; 2D Transistion Metal Dichalcogenides; WS2; WSe2; Flexible Electronics
- Abstract:
- Novel materials, including zinc oxide (ZnO) and 2D transition metal dichalcogenides (TMDs), have been investigated in this dissertation for the realization of high-performance large-area integrated circuits. These novel materials may provide differential advantages over the established large-area thin film technology based on silicon, which has been extensively employed in applications such as large-area flat panel displays, high-speed active matrix thin film circuits, flexible and wearable electronics, etc. The dissertation begins with the discussion of high-performance plasma-enhanced atomic layer deposition (PEALD) of ZnO thin films and ZnO thin film transistors (TFTs) with a field effect mobility of ~ 10 to 20 cm2/Vs, which have been demonstrated. Offset-drain ZnO TFTs, which are able to withstand or switch voltage beyond 80 V, have also been demonstrated. These results shed light on the realization of large-area active-matrix circuits beyond the capabilities of the current display industry where high circuit speed or high operation voltage is required. To further improve the performance of ZnO-based electronics, many related materials, including doped ZnO, zinc nitride, and aluminum nitride, have been investigated. Doped ZnO has been proposed as the carrier injection layer that can improve the conductivity of metal-semiconductor contact in ZnO TFTs. Aluminum-doped ZnO thin films have been deposited using triisobutyl aluminum (TIBA) as the dopant precursor instead of trimethyl aluminum (TMA) in order to improve the uniformity of dopant distribution because TIBA has much lower vapor pressure than TMA. AZO thin films with resistivity ~ 10-2 Ω·cm have been achieved by PEALD. Besides, aluminum nitride and zinc nitride thin films have also been studied using PEALD. In addition to the showerhead PEALD system, a novel inductively coupled plasma ALD system has been designed and set up that provides RF power up to 500 W in order to generate a highly reactive nitrogen plasma source and enable the deposition of high-quality metal nitride at relatively low temperature. These metal nitride thin films may provide additional building blocks to enhance the speed and thermal stability of ZnO-based thin film devices and circuits. Owing to their excellent electrical and mechanical properties, 2D-TMD thin films have been studied for flexible electronics applications. High quality MoS2 and WS2 thin films have been achieved via mechanical exfoliation and chemical vapor deposition. To fabricate MoS2- and WS2-based TFTs, a 5-step device fabrication process has been developed, which is compatible to both the conventional rigid substrate and the ~ 4.8 nm thick solution-cast polyimide (PI) flexible substrate. The MoS2 and WS2 TFTs fabricated on PI substrate exhibit a field effect mobility of between 1 to 20 cm2/Vs, which is similar to that of those fabricated on rigid silicon substrate. More importantly, extraordinary mechanical strength and stability have been demonstrated for MoS2 and WS2 TFTs fabricated on PI substrate. A reasonably small degradation in device performance has been observed in these flexible 2D-TMD TFTs under static bending to the radius of ~ 2mm and after cyclic bending up to 100,000 cycles. Finally, attempts to create integratable 2D-TMD circuits have been demonstrated. To realize large-area 2D-TMD based circuits, growth of wafer-scale continuous WSe2 thin films has been demonstrated using metal organic chemical vapor deposition (MOCVD). Deposition has been achieved at as low as 400 °C, which allows deposition on glass and polymeric substrate and enables the transfer-free fabrication of WSe2 TFTs and circuits on arbitrary platforms. Patterning and post-growth thickness modulation of continuous WSe2 thin film have been demonstrated using CF4 plasma and O2 plasma, whereby high-speed etching and nanometer-scale film thinning can be realized. With the capability of depositing and patterning wafer-scale WSe2 thin films, an array of p-channel WSe2 TFTs have been fabricated with a field effect mobility of ~0.01 cm2/Vs and an on-off ratio greater than 104.